1. Field of the Invention
The present invention relates to a method for fabricating a titanium nitride thin film by CVD, and to a CVD apparatus therefor.
2. Related Art
Reactive sputtering methods featuring the use of metallic titanium targets and nitrogen gas have been primarily used in the past for growing titanium nitride thin films on the substrates of semiconductor devices, various electronic components, various sensors, and the like. In recent years, due to the creation of superfine, large-scale-integration silicon circuits, the design rules for DRAM of 64 Mbit and higher, require dimensions of about 0.35 .mu.m or less, and cause the aspect ratio of contact holes in devices to gradually increase. A drawback of using titanium nitride thin films as barrier metals for such contact holes is that inadequate step coverage results when these titanium nitride thin films are deposited by conventional reactive sputtering. Unsatisfactory step coverage adversely affects the electrical characteristics of contact holes and is expected to pose a serious problem for creating the next generation of devices. It is therefore expected that conformal barrier metals will be formed using CVD techniques, which provide excellent coverage characteristics or filling characteristics.
In view of this situation, attention was attracted in recent years to techniques for fabricating titanium nitride thin films by CVD (chemical vapor deposition). Various CVD techniques and source gases have been proposed for fabricating titanium nitride thin films. One such technique features the use of a tetrakis(dialkylamino)titanium (hereinafter abbreviated as "TDAAT"), an organometallic compound. The chemical structural formula of TDAAT is shown in FIG. 8. In the chemical structural formula, R is an alkyl group. Tetrakis(diethylamino)titanium (hereinafter abbreviated as "TDEAT") is obtained when R is an ethyl group, and tetrakis(dimethylamino)titanium (hereinafter abbreviated as "TDMAT") is obtained when R is a methyl group.
These organotitanium compounds, although liquids at room temperature and atmospheric pressure, can be fed into a reaction vessel through a shower head together with carrier gases such as H.sub.2, argon, and N.sub.2 when vaporized. A mixed gas (such as added gas) chemically reactive with organotitanium compounds is also fed into the reaction vessel. A substrate is disposed within the reaction vessel, and this substrate is kept at a prescribed reaction temperature.
The organotitanium compound and the mixed gas initiate a reaction that yields titanium nitride, which adheres to the substrate as a titanium nitride film. It is known that the step coverage and electrical characteristics of the titanium nitride thus deposited depend on the reaction pressure, the substrate temperature, and the flow rates of the mixed gas and the organotitanium compound reacting inside the reaction vessel.
For example, according to Raajimaker (Thin Solid Films, 247 (1994), 85) and other sources, the source gas TDAAT is fed to a reaction vessel together with argon (carrier gas), and a titanium nitride thin film is fabricated using ammonia gas (NH.sub.3) as an added gas. The flow rate of the ammonia gas is 1000 sccm or greater. The resulting titanium nitride thin film delivers adequate step coverage (85%) when deposited in a contact hole having a diameter of 0.8 .mu.m with an aspect ratio of 1. Applied to contact holes with diameters of 0.35 .mu.m, and less, such as those used for 64-Mbit DRAMS, however, this technology is expected to yield a step coverage of merely 20% or less.
Furthermore, according to Jackson et al. (R. L. Jackson, E. J. MCineney, B. Roberts, J. Strupp, A. Velaga, S. Patel and L. Halliday, Proc. Advanced Metallization for ULSI Application, ed. by D. P. Favreau, Y. Shacham-Diamond, and Y. Horiike (Mat. Res. Soc., Pittsburgh, Pa., 1994), p. 20), a source TDEAT is vaporized by being passed through a vaporizer, and the vaporized material is fed together with a carrier gas (nitrogen gas) to a reaction vessel through a shower head. In addition, ammonia gas (NH.sub.3) is added through a shower head in a separate conduit, yielding a titanium nitride thin film. In particular, the cited paper reports on the effect demonstrated by the ratio of the feed rates of the source TDEAT and the ammonia gas. The step coverage decreases from 65% to about 20% as the amount in which the ammonia gas is added increases at a film depositing temperature of 350.degree. C. and a pressure of 10-50 torr in a contact hole with a diameter of 0.35 .mu.m and an aspect ratio of 3.4. On the other hand, the step coverage for the same contact hole decreases to 5% if the film-depositing temperature is raised to 425.degree. C. Thus, the step coverage of fine contact holes becomes inadequate if the flow rate of ammonia gas increases in relation to TDEAT.
Furthermore, Intermann et al. (A. Intermann and H. Koerner, J. Electrochem. Soc., Vol. 140, No. 11 (1993), 3215) published a detailed report on the reasons for the deterioration of step coverage due to the addition of such ammonia gas, concluding that step coverage deteriorates as a result of violent chemical reactions between TDMAT and the ammonia gas.